Re: IBM disk arm reduction "rule of thumb"

[daparnin@xxxxxxxxxxxxxx]

I think that you may have hit on it--The "rule of thumb" is "the more cache
the better".  This is probably true up to a point.  It's been a year or two
since I built my last PC so my cache values might be a bit off but in the
PC world I've read drive reviews that compared the same drive with 4MB of
cache to one with 8MB.  The 8MB would blow away the 4MB.  If I recall there
is a point of diminishing returns for the $$$ spent on cache.  Given that
the iSeries tends to balance the data between drives I would think that the
optimum cache memory per drive would be a function of the drive capacity,
percentage used, and perhaps the number of drives.  If you throw in things
like RAID, mirroring, etc. the formula would probably get even more
complex.  Still, no matter what you buy there will be something bigger,
faster, and cheaper available next week.


Dave Parnin
--
Nishikawa Standard Company
Topeka, IN  46571
daparnin@xxxxxxxxxxxxxx





                                                                                
                                 
                      Al Barsa                                                  
                                 
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Unfortunately, I don't know of a rule of thumb.

However, this is why IBM is adding faster drives, and more cache on the
IOAs.

Al

Al Barsa, Jr.
Barsa Consulting Group, LLC

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Hello All,

Does anyone know if IBM has a "rule of thumb" on the acceptable level of
disk arm reductions?  We are in the process of developing a configuration
for a new 520.  Our current 820 has 18 disk arms.  One vendor suggests
reducing to 8 in the main box (no tower).  Another vendor recommends 12
utilizing a tower.  The reasoning is that at our current peak disk
accesses, we would need that many to keep the disk ops/second at 60 or
below.   Both quotes are for 4326 35G/15K disk units.

All comments/suggestions would be most appreciated.
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